1. Field of the Invention
The present invention concerns a magnetic resonance apparatus of the type having an RF antenna unit, a gradient coil unit and an RF shield, the conductor structures of the gradient coil unit and the RF shield being independent of one another, the RF shield being arranged between the RF antenna unit and the gradient coil unit; with an RF field return (or field reflux) space existing between the RF antenna unit and the gradient coil unit, the RF field reflux space closing the RF magnetic field lines of the RF antenna unit and being bordered by the RF shield on the side of the gradient coil unit.
2. Description of the Prior Art
Magnetic resonance technology is a known modality for examination of subjects, among other things for the acquisition of images of the inside of a subject. Rapidly-switched gradient magnetic fields that are generated by a gradient coil system and serve for spatial coding are superimposed on a static basic magnetic field that is generated by a basic field magnet in an examination region of a magnetic resonance apparatus (MR apparatus, for example for MR tomography or MR spectroscopy). Furthermore, the MR apparatus has one or more radio-frequency antennas (RF antennas), one of which radiates RF signals into the examination subject to excite MR signals (usually a whole-body RF antenna) and another one (or more) of which acquires the emitted MR signals (usually a local antenna). One or more magnetic resonance images are generated based on the MR signals.
A MR apparatus in which eddy currents are compensated is known from U.S. Pat. No. 4,864,241. This ensues using gradient coils divided into two parts that typically form a hollow-cylindrical unit. An RF antenna with a smaller radius, likewise fashioned as a hollow cylinder, is introduced into the gradient coil unit for RF field generation.
From DE 44 14 371 A1, an MR apparatus is known in which a radio-frequency shield is arranged between the radio-frequency antenna and the gradient coil system, the radio-frequency shield being designed so that it is permeable for the electromagnetic fields generated by the gradient coil system in the low-frequency range and is impermeable for the fields generated by the radio-frequency antenna in the radio-frequency range.
Such a designed and allocated radio-frequency shield has the effect that the space lying between RF shield and radio frequency antenna unit forms an RF field return space, i.e. for closing of RF magnetic field lines. The RF magnetic field lines (ideally) proceed homogeneously in the acquisition region of the MR apparatus and are closed by the RF field return space. A high magnetic energy density that is too high in the RF field return space leads to disadvantageous interactions with, for example, the RF shield and increases the radiated power loss of the RF antenna unit.
A radio-frequency antenna of an MR apparatus can be designed, for example, as a type of antenna known as a birdcage antenna. A birdcage antenna is designed for generation of a homogeneous radio-frequency field within a volume normally enclosed by it, with longitudinal conductors that are connected with one another by ferrules are arranged parallel to one another and equally spaced on an imaginary cylinder surface. Tuning ensues in the high-pass and low-pass filter ranges, by fixed capacitances connected in each of the conductors, or in the ferrules between the conductors, so that a homogeneous radio-frequency field results given resonant excitation. Embodiments of such a birdcage antenna are described, for example, in U.S. Pat. No. 4,680,548. The radio-frequency antenna also can be designed as an array antenna. The array antenna is formed by a number of essentially similar, mutually overlapping conductor loops. Embodiments of such an array antenna are described, for example, in U.S. Pat. No. 4,825,162. Such antennas also can be designed multi-staged, meaning that a number of antennas can be arranged next to one another and abutting one another.
From DE 42 30 145 A1, an MR apparatus is known that has a basic field magnet that allows a transversal access to the measurement volume. The MR apparatus has a gradient coil system with axially-separated segments. An axial RF coil system that can be inserted into an axial bore of a supporting body or transversally into the cavity of the basic field magnet is used for generation of an essentially homogenous RF field in the measurement volume. The MR apparatus, or the components thereof (such as the basic field magnet, the gradient coil system and the RF coil system) are designed with a view toward achieving an optimally large lateral access to the measurement volume for simplifying the implementation of therapy measures such as microsurgical procedures.
A transversal gradient coil arrangement is known from DE 44 22 782 C2 in which, in the gradient coils system, windings of the primary coil and the secondary coil that are farther from the center in the axial direction of the gradient coil arrangement exhibit a smaller radial interval relative to one another than windings lying closer to the center. This arrangement is intended to achieve an improved homogeneity of the magnetic field in the examination volume with the simultaneous possibility of shortening the coil. DE 44 22 782 C2 additionally describes a method for calculation of the conductor curve of such a gradient coil.
A sub-divided gradient coil unit for a magnetic resonance apparatus is known from WO 97/35214.
A magnetic resonance apparatus is known from DE 102 46 308 A1 having a hollow opening in which a gradient coil system, divided into two parts, is mounted for generation of gradient fields. The gradient coil system has two hollow-cylindrical halves between which is arranged a specially designed antenna system for transmission of radio-frequency signals and acquisition of magnetic resonance signals.
DE 198 51 584 C1 describes a gradient coil unit in which conductors of a primary gradient coil are electrically connected with conductors of the secondary gradient coil.
The technical development of MR apparatuses has arrived at a stage of maturity that places in the foreground, as a next step, the development of more compact systems with reduced cost. The greatest potential for cost reduction is to try to reduce the volume (diameter reduction) of the most complex component of the system, the basic field magnet. In order to enable this while still maintaining an adequately large acquisition volume, it is necessary to generate the dynamic fields of the MR apparatus (gradient and RF fields) by means of components that occupy as small a volume as possible.